Arctic browning: extreme events and trends reversing arctic greening

(1)

E D I T O R I A L C O M M E N T A R Y

Arctic browning: extreme events and trends reversing arctic greening

G A R E T H K . P H O E N I X¹ and J A R L E W . B J E R K E²

1Department of Animal and Plant Sciences, University of Sheffield, Western Bank, Sheffield S10 2TN, UK,²Norwegian Institute for Nature Research (NINA), FRAM–High North Research Centre for Climate and the Environment, PO Box 6606 Langnes, NO-9296 Tromsø, Norway

NOAA’s recent assessment of Arctic greenness has reported a remarkable finding: the Arctic is browning (Epsteinet al., 2015). Whilst a clear greening trend has been apparent for most of the satellite record’s 33 year history (indicating an increase in biomass and productivity), there is now an overall decline in greenness from 2011 to 2014. If this is a new direction of travel for arctic vegetation, rather than just a temporary depar- ture from long-term greening, this has major implica- tions for not only our understanding of the future of arctic vegetation, but also arctic carbon, nutrient and water cycling, surface energy balance and permafrost degradation, and therefore feedback to climate, all of which are strongly influenced by vegetation composi- tion, productivity and biomass. The urgency in understanding what is happening here is clear. Most models predict arctic greening; to what extent are they wrong, and why?

Arctic greening has rightly received much attention.

Satellite and observational data have consistently con- firmed an increase in vegetation cover and productivity in many regions (Xu et al., 2013) caused most notably by the expansion of large stature deciduous shrubs (Myers-Smith et al., 2015). Likewise, field simulation experiments provide strong evidence that greening is driven by warming (Elmendorfet al., 2012). However, the magnitude of the recent browning is large and can-

not be ignored. For both the Eurasian Arctic and the Arctic as a whole, Epstein et al. report the 2014 maxNDVI (greenness) to be below the 33-year average.

To find lower values than 2014, you have to go back to 1996 for the whole Arctic and to 1993 for the Eurasian Arctic. However, while browning is the overall trend, there is considerable regional variation and the Arctic is not browning everywhere. These findings raise important questions that represent priority challenges, including (1) what is driving the browning? (2) is browning the new trajectory or only a temporary reversal of greening? and (3) what arctic regions and vegeta-

(a)

(e) (c)

(d) (b)

(f)

etanimod sdnerTetanimod stnevEegnahc ten elttiL

Greatest greening from trend change Intermediate greening from trend change Baseline greenness

Browning from trend change

Browning from events (large, temporary browning) Fig. 1 Scenarios for arctic browning and greening with trend

and event drivers. Boxes represent a landscape unit or region (e.g. edge size of magnitude 10–100 km) with each ‘pixel’ a vegetation or landscape unit within it. Each subsequent box moves on 10 years (allowing recovery from browning events). Scenar- ios where trends dominate: (a) trend greening; (b) browning driven by trend climate (e.g. reductions in summer warmth index/

increased snow cover duration). Scenarios where events dominate: (c) browning from spatially and temporally discreet events combine to result in net browning–in this scenario, against a background of trend greening in nondamaged areas; (d) max browning – spatially and temporally discreet events add to trend browning. Scenarios where little or no net change occurs despite drivers operating: (e) trend greening and trend browning largely balanced; (f) trend greening and event browning largely balanced.

Global Change Biology (2016), doi: 10.1111/gcb.13261

(2)

tion types are most sensitive so might show the greatest browning in future?

Browning can be caused by a reversal of greening drivers. As raised by Epsteinet al. (2015), declines in greenness in some regions, especially early season, arise from greater and longer snow cover (Bienieket al., 2015), and an earlier analysis showed the declining greening of the Eurasian arctic to be linked to reduced summer warmth index (Bhattet al., 2013). If so, against an ongoing trend of some of the greatest warming on the planet and declining snow cover duration (AMAP, 2011), we might expect greening to resume soon, although a warmer arctic will result in some areas receiving more snow and having longer snow cover duration (AMAP, 2011). Equally, changes in the ‘trend’

drivers of greening may not explain the widespread extent of browning, and other mechanisms will apply.

In particular, while warmer growing seasons (i.e.

trend climate change) may be the dominant driver for greening, a number of lines of recent evidence show that extreme events and winter warming drive browning. Critically, both extreme events and winter temperatures are also increasing as part of climate change, and in the case of winter temperatures, increasing much more so than summer (AMAP, 2011).

So, despite having the greatest rates of warming glob- ally, the concurrent increases in browning drivers that come with climate change means that we can be far

less certain about ongoing greening (example browning and greening scenarios are shown in Fig. 1). For instance, tundra fire can cause complete loss of vegetative cover, and tundra fire frequency is expected to increase with climate change (Bret-Harte et al., 2013). Thermokarst development with permafrost degradation can lead to browning where thaw features expose ground or create water bodies (although succession will re-green these). In the record-low productivity for north-west Scandinavia observed in 2012, 14 different types of anomalous weather events were detected that drove browning (Bjerkeet al., 2014). While these occurred further south than the study area of Epstein et al., they may well represent an early warning for higher latitudes as they warm. The best studied of these is currently extreme winter warming which leads to mid-winter bud burst and loss of freeze tolerance (Bokhorst et al., 2011), although the numerous other drivers identified range from warm autumns reducing winter hardening, rain- on-snow resulting in plant ice encasement, lack of snow cover combined with high irradiance leading to frost drought (Fig. 2), through to snow falling on unfrozen ground enhancing snow mould growth and respiratory losses from subnivean plants. In addition, outbreaks of defoliating insects and rust fungi were also cited as browning drivers. Furthermore, events can interact to drive greater browning, such as frost

(a) (b) (c)

(d) (e) (f)

Fig. 2 Browning events: (a, b) extreme winter warming damage toEmpetrum nigrumheathland in Sub-Arctic north-west Scandinavia;

(c) icing damage toDryas octopetalain High Arctic Svalbard (Saxifraga oppositifoliaflowering); (d, e) frost drought damage toCalluna vul- garisheathland in central Norway; and (f) fire on flammable, frost drought killed,Callunaheathland resulting in soil exposure and ero- sion. Photo credits: (a, b) Stef Bokhorst, (c) Rachael Treharne, (d, e, f) Gareth Phoenix.

2 E D I T O R I A L C O M M E N T A R Y

(3)

drought damaged heathland becoming more fire prone (Fig. 2f). However, while this may suggest the Arctic faces an onslaught of browning, vegetation also shows evidence of rapid recovery from these events (vegetation in the extreme winter warming and fire studies recovered in 2–4 years; Bokhorst et al., 2011;

Bret-Harteet al., 2013).

And here lays one of the main challenges. Events are hard to study (and those that occur in winter even more so). The sporadic nature of events in time and space means they cannot be predicted beforehand, infrastruc- ture cannot be set up in advance in the right place and time to monitor an upcoming event, and when they do occur, it may be the aftermath of damage that allows us to detect the event, hence missing the opportunity to study it in progress. The damage is also often transient, with ecosystems recovering after a few years. So, while greening may be viewed more as a steady and gradual process, browning may come from a large number of different drivers, either as trend change but also often as biotic or weather events, reducing greenness in different parts of the landscape, at different times, tem- porarily. Net browning will arise if the sum of these in space and time overrides greening. Predicting that from a large number of events that we still poorly under- stand is a real challenge.

Acknowledgements

GKP and JWB are supported by the Leverhulme Trust (UK) (grant F/00 118/AV), the Research Council of Norway (project

no. 216434 and 225006) and the Natural Environment Research Council (UK) (grant NE/L002450/1).

References

AMAP (2011)Snow, Water, Ice and Permafrost in the Arctic (SWIPA): Climate Change and the Cryosphere. Arctic Monitoring and Assessment Programme (AMAP), Oslo, Nor- way. xii + 538 pp.

Bhatt US, Walker DA, Reynolds MKet al. (2013) Recent declines in warming and vegetation greening trends over pan-arctic tundra. Remote Sensing, 5, 4229–4254.

Bieniek PA, Bhatt US, Walker DAet al.(2015) Climate drivers linked to changing seasonality of Alaska coastal tundra vegetation productivity.Earth Interactions,19, 1–29.

Bjerke JW, Karlsen SR, Høgda KAet al.(2014) Record-low primary productivity and high plant damage in the Nordic Arctic Region in 2012 caused by multiple weather events and pest outbreaks. Environmental Research Letters, 9, 084006.

Bokhorst S, Bjerke JW, Street LE, Callaghan TV, Phoenix GK (2011) Impacts of multiple extreme winter warming events on sub-Arctic heathland: phenology, reproduction, growth, and CO2 flux responses. Global Change Biology, 17, 2817–2830.

Bret-Harte MS, Mack MC, Shaver GRet al.(2013) The response of Arctic vegetation and soils following an unusually severe tundra fire.Philosophical Transactions of the Royal Society B: Biological Sciences,368, 20120490.

Elmendorf SC, Henry GHR, Hollister RDet al.(2012) Global assessment of experi- mental climate warming on tundra vegetation: heterogeneity over space and time.

Ecology Letters,15, 164–175.

Epstein HE, Bhatt US, Raynolds MKet al.(2015) Tundra Greenness. In:Arctic Report Card: Update for 2015(eds Jeffries MO, Richter-Menge J, Overland JE). NOAA, Sil- ver Spring, MD. Available at: http://www.arctic.noaa.gov/reportcard/ (accessed 18 Dec 2015).

Myers-Smith IH, Elmendorf SC, Beck PSAet al.(2015) Climate sensitivity of shrub growth across the tundra biome.Nature Climate Change,5, 887–891.

Xu L, Myneni RB, Chapin FS IIIet al.(2013) Temperature and vegetation seasonality diminishment over northern lands.Nature Climate Change,3, 581–586.

E D I T O R I A L C O M M E N T A R Y 3